Drawing on the testimony of Syrian refugee children, the charity calls on the United Nations to increase its presence on the ground.

It says almost every child it spoke to has seen a family member killed.

The report comes at the start of the UN General Assembly's annual meeting.

Earlier, UN envoy Lakhdar Brahimi said the situation in Syria was "extremely bad and getting worse".

Speaking after he had briefed the Security Council following his first visit to Syria since taking up the post, Mr Brahimi admitted he did not have a full plan on how to bring peace.

Although Syria is not formally on the agenda at the UN General Assembly's annual conference, it is expected to dominate discussions.

'Systematic abuse'

Save the Children has documented numerous cases of abuse against children during Syria's 18 month conflict, which was sparked by unrest over the arrest and torture of children in the town of Deraa.

The children had written a well-known slogan of the popular uprisings in Tunisia and Egypt on a wall.

One 15-year-old told the charity he had cigarettes put out on him when he was imprisoned in what used to be his school.

Another described being given electric shocks and sharing a cell with decomposing bodies, while a third teenager, Wael, said he had seen a six-year-old die after being tortured and starved.

The 16 year-old told the report's authors: "I watched him die. He only survived for three days and then he simply died."

"He was terrified all the time. They treated his body as though he was a dog."

Save the Children's report described the acts as "consistent, recurring and appalling".

Cat Carter, a spokeswoman for the charity, said: "The stuff I've heard from children is absolutely appalling. I've heard of children as young as 10 being tortured. I've heard of children, as young as eight, helping to remove dead bodies from rubble, with their own hands."

The UK-based charity called on the UN to increase its presence on the ground to document violations so that perpetrators can be brought to account.

'No immediate prospect'

Speaking at the UN headquarters in New York on Monday, Mr Brahimi said: "There is no disagreement anywhere that the situation in Syria is extremely bad and getting worse, that it is a threat to the region and a threat to peace and security in the world."

He said there was "no prospect for today or tomorrow to move forward", but added "I think that we will find an opening in the not too distant future".

Fighting is continuing in Syria's second city, Aleppo

The BBC's Barbara Plett at the UN says Mr Brahimi's mission to forge a political solution seems almost impossible, with the two parties intent on fighting rather than talking, and the Security Council deeply divided.

The UN says more than 20,000 people have been killed since anti-government protests began in Syria in March 2011. Activists put the death toll as high as 30,000.

Violence continued across the country on Monday, with activists saying the government had bombed parts of the second city, Aleppo.

The Local Co-ordination Committees, an opposition activist network, reported that at least 40 people had died in fighting, including 13 in Aleppo.

The group said the victims included three children from one family killed in air strikes in the central district of Maadi.

In addition to the fighting in Aleppo, anti-government activist group the Syrian Observatory for Human Rights reported that a five year-old girl and a man were killed during the bombardment of the southern town of Dael, in Deraa province.

Clashes between government forces and rebels were also reported in the north-eastern and north-western districts of the capital, Damascus.

More than 260,000 Syrians have fled to neighbouring countries, the UN says. There are also thought to be more than 1.2 million internally displaced people, and 2.5 million in need of humanitarian assistance.